Real-time x-ray analysis roots out production flaws

At Raytheon's Reliability Analysis Lab (RAL), our goal is to discover root causes for product flaws or malfunctions. Customers—representing other Raytheon divisions, government organizations, and private companies in the defense, telecommunications, aviation, and automotive industries—approach us with a faulty component, product, assembly, or even process that they can't correct.

For example, one client presented the lab with a train motor assembly—a dense and complex structure that assists in controlling and maneuvering a communications dish. The assembly is designed to allow the dish to rotate 360° both clockwise and counterclockwise. The client, however, discovered that some units would not operate through the full 360° rotation, while others would not operate at all.

As a first step in understanding failure mechanisms in such a system, RAL engineers explore the application background with the client. The parties discuss every aspect of production, including the production-line setup, the materials used, and the inspection method that determines product quality. Customers generally dictate the depth and scope of an investigation, but our engineers select investigation methods that they think will reveal the failure's root cause most effectively. A typical inquiry might begin with nondestructive inspection and proceed to more destructive methods if the results remained inconclusive.

For the train motor assembly, the client explained the assembly's structure and parts and provided both failed and problem-free units for comparison. The train motor assembly is comparatively large and dense, so our engineers immediately ruled out ultrasonic inspection as a viable option. Because it is a sealed assembly, they ruled out visual inspection as well. To view the inner parts of the sample without destroying it, the engineers pursued real-time x-ray inspection.

Real-time x-ray inspection equipment, like the custom-engineered system in Figure 1, represents the first line of attack when the engineers are faced with a sample's unknown inner architecture. X-ray techniques "see" component dimensions as well as inner-structure placement, and they permit cross-sectioning of specific areas of interest, penetrating into dense objects for defect identification. With a real-time system, engineers can also examine samples in motion. The ability to inspect a sample thoroughly in this one step often eliminates the need for further tests.

For a real-time system, an x-ray source remains activated throughout the inspection process. The sample, mounted on a six-axis computer-numerical-control (CNC) manipulator (Figure 2), moves on its x-, y-, and z-axes to achieve views at any angle within the x-ray beam. As the x-rays penetrate the sample, x-ray photons strike the cesium-iodide input transducer of an image intensifier, which converts them into light photons. A photo cathode in turn converts the light photons into photo-electrons. These photo-electrons are accelerated and focused onto an anode output phosphor, producing a camera image. A video monitor can display either a positive or negative version of the image.

A key advantage of real-time x-ray inspection is that the geometry of the x-ray system can be used to magnify images. A focused stream of high-energy electrons released from an energy source (Figure 3) strikes a target, producing the x-ray beam. Moving the x-ray source closer to the sample relative to the image intensifier magnifies the image. The clarity of this magnified image depends on the size of the focal spot. The smaller the focal spot, the clearer the image produced, permitting higher levels of magnification.

The alternative to real-time x-ray inspection requires that engineers analyze images captured on wet film. With this method, thoroughly viewing any sample requires taking many static images at multiple angles. Although still widely used, the wet-film method is giving way to real-time x-ray alternatives because of the time required to take, develop, and evaluate images, which delays defect-detection and production-process correction.

The RAL includes a lead room that serves as an x-ray chamber, and the lab also has its own imaging chain, which consists of a 9-in. triple-field image intensifier, CCD camera, and video monitor for image capture and viewing. Designed and installed by members of the Feinfocus (Stamford, CT; www.feinfocus.com) engineering team, this custom system is actually a walk-in booth. An operator can enter and place the sample, then exit and secure the door. An independent LCD controller in an adjoining room controls the tube. The manipulator has its own computer that allows it to record measurements using its axis encoders at a resolution of 0.01 in.

RAL engineers placed the train motor assembly in the x-ray chamber and inspected it both while it was motionless and while it was in use. Engineers closely examined lead-wire placement, ensuring that leads connected properly to the motor brushes and noting whether any wires came into contact with a lug-nut washer in an inner assembly. Tests of failed parts revealed that in some cases wires were attached incorrectly or were interfering with other parts, but the engineers could identify no single cause. The breakthrough came when they viewed the assembly in motion. Manipulating and viewing the part from every angle as the assembly's spindle moved, the x-ray inspection system revealed that lead wires touched the lug-nut washer when the assembly vibrated.

In addition to identifying all areas of possible contact between wires and the lug-nut washer, our final report on the train motor assembly recommended ways in which better quality workmanship could prevent assembly failures. Although this application is too recent to permit calculation of a quantitative return on investment, RAL's parent, Raytheon, has proved the advantages of real-time x-ray inspection quantitatively. Raytheon reports that the system—by screening out faulty units before they ship to customers—has already paid for itself in money and time savings since its installation in 2000.

Titus, Jon, "A tale of three labs," Test & Measurement World, March 2003. p. 10. www.tmworld.com/archives.